Inductive coin testing apparatus utilize air core or ferrite core coils as sensing devices to measure the various electrical and physical parameters of coins. A coin is tested by detecting the effect of the coin on an alternating electromagnetic field produced by the coil. At a given frequency, this effect depends upon the coin's diameter, thickness, conductivity and permeability. An effect on this field causes a corresponding effect upon the coil's impedance which may be measured using various techniques.
The extent to which an electromagnetic field penetrates a coin decreases with increasing frequency. Therefore, as frequency increases, the physical properties of material near the surface of a coin have a greater effect on the field (and the coil's impedance), and interior material and coil thickness have a lesser effect. This phenomenon is especially significant when testing for laminated coins such as the United States ten and twenty-five cent coins.
Most high quality coin identificaton mechanisms, capable of excellent slug and foreign coin rejection, use multiple sensors in order to effectively measure physical characteristic, such as thickness and diameter, and material properties, such as conductivity and permeability. These mechanisms generally undertake several measurements which may require as many as five separate ferrite core coils. According to the method of one mechanism, first a mixed measurement is made that is dependent upon thickness, diameter and material. However, in order to separate these variables and establish coin identity, two additional measurements are made, one specific to thickness and a second specific to material.
Where a single sensor has been used, it generally has performed only a single test function.